The present invention relates to a screen device for separating fiber suspensions, comprising a screen housing, a rotor situated centrally in the screen housing, a drive motor for rotating the rotor about a rotor axis, and a tubular screen concentrically surrounding the rotor and dividing the interior of the screen housing into a central chamber for receiving a fibre suspension to be separated and an outer accept chamber for receiving an accept fraction of the fibre suspension which has passed through the screen. A first rotor portion of the rotor and a first axial screen portion of the screen define a first screen chamber of the central chamber, and a second rotor portion of the rotor and a second axial screen portion of the screen define a second screen chamber of the central chamber. The screen device further comprises an inlet member for supplying the fibre suspension to be separated to the central chamber, an accept outlet member for discharging the accept fraction from the accept chamber, a first and a second reject outlet member, respectively, for discharging a reject fraction, which has not passed through the screen, from the first and the second screen chamber, respectively, first pulsation elements arranged on the first rotor portion and extending along the first axial screen portion, and second pulsation elements arranged on the second rotor portion and extending along a second axial screen portion, the pulsation elements for subjecting the fibre suspension to pulses close to the screen during rotation of the rotor.
Such a screen device, preferably a so called closed screen, is preferably used for separating pulp suspensions, for instance for fractionating fibres or separating contaminants, such as undesired particles, undefibrated material or fibre bundles. A screen device of this kind is required to be efficient, i.e. to produce an accept fraction containing as much good fibres as possible of the fibres that existed in the original fibre suspension, or in other words to produce a reject fraction containing few good fibres, preferably none at all. Also, the screen device should have a low power consumption, be space saving, be inexpensive and be service-friendly.
A problem often encountered for instance when screening paper pulp suspensions is that the consistency of the supplied suspension varies greatly. On one hand a low fibre concentration leads to a greater hydraulic load on the screen. On the other hand, a high fibre concentration requires a greater supply of energy for the operation of the screen device.
Further problems may also arise if the flow of the supplied fibre suspension varies. Thus, in the case of varying suspension flow and high fibre concentration, a too strong thickening of the fibre suspension may easily arise between the inlet member and the reject discharge members. Such a thickening of the fibre suspension limits the capacity and efficiency of the screen device, since the screen will become partly blocked by a tight fibre network. Thus, an increase in the fibre concentration has the consequence that the strength of such a fibre network formed on the screen would increase, so that the pulsation elements of the rotor would not be fully capable of dissolving the fibre network. When separating a fibre suspension having a relatively high concentration, say about 3.5%, a small increase in concentration would give rise to a large increase in the energy required for providing fluidization and breakdown of the fibre network. This has the consequence that it will be more difficult to accomplish optimal separation of high consistency suspensions than of low consistency suspensions.
Since the energy supplied by the rotor is constant along the entire extension of the screen, the fibre concentration of the fibre suspension supplied to the inlet end of the tubular screen has to be low enough so that the thickening of the fibre suspension will not be too high at the end of the screen which is opposite to the inlet end. This may result in that the energy supplied to the incoming fibre suspension will be too high causing too much fluidization of the suspension.
In order to accomplish a satisfactory separation of a high consistency fibre suspension, in certain types of screen devices the rotor previously has been provided with broad and extended pulsation elements producing prolonged suction pulses on the screen, so that a portion of the liquid which has passed through the screen into the accept fraction chamber is regained to the screen chamber. A problem in this connection however, is that the separation operation is more sensitive to disturbances with increasing consistency. To counteract disturbances when separating such high consistency fibre suspensions, the rotor has to be driven at a rotational speed that is faster than that required for separating a low consistency fibre suspension having a substantially non-fluctuating fibre concentration.
To remedy the above separation sensitive problem it has previously been suggested to divide the incoming suspension stream into two part streams, which are distributed along two relatively short portions of the screen, as seen in the axial direction, with the result that the thickening axial distance for each part stream will be relatively short. For instance, U.S. Pat. No. 4,328,096 discloses a screen device comprising a closed tank, and a vertical cylindrical screen arranged in the tank. Inside the screen there is a rotor driven by a drive motor situated under the tank. The rotor comprises a plurality of angled wings, the function of which is to split the suspension stream coming into one end of the cylindrical screen into two separate part streams, which are to flow in opposite directions in a common screen chamber along the screen. A drawback to this known screen device is among other things that the wings cannot generate any pressure or suction pulses on the screen for regaining liquid from the accept fraction chamber back to the screen chamber. This means that the fibre suspension consistency has to be relatively low. A further drawback is that the intended split of the suspension stream into said two separate part streams is difficult to achieve. It should be most unlikely that a sharp separation of the part streams is achievable. Probably, the two part streams will disturb each other already at relatively small deviations from the intended conditions (for instance fluctuations of the fibre concentration), which will disturbe the separation operation.
U.S. Pat. No. 5,318,186 discloses a screen device provided with an inlet member arranged at the middle of a cylindrical screen, for distributing incoming fibre suspension into two part streams having opposite axial flow directions in a common screen chamber. A serious drawback to this known screen device is that it requires a relatively large space because of the localisation of the inlet member. In addition, the distribution of the incoming suspension stream will probably give rise to part streams of different sizes.
The object of the present invention is to provide a screen device of the kind above discussed, which eliminates the above presented problems of the known screen devices.
This object is obtained by means of a screen device of the kind initially described, which is characterized by distribution means for dividing the fibre suspension supplied by the inlet member into two part streams having the same axial directions in relation to the rotor and for distributing the two part streams to the first and second screen chambers, respectively.
Preferably, the distribution means comprise the first and second rotor portions, which are formed with tubular coaxial walls for distributing the incoming fibre suspension from the inlet member via the interior of the tubular walls to the first and second screen chambers.
As an alternative, the distribution means may be arranged to distribute the two part streams of the fibre suspension directly to the respective screen chambers via two separate inlets, the rotor portions being designed so that the screen chambers do not communicate with each other. This alternative enables a two stage separation of the fibre suspension.
According to an embodiment of the invention the tubular wall of the second rotor portion surrounds and extends axially along the tubular wall of the first rotor portion. The tubular wall of the first and the second rotor portion, respectively, suitably has the shape of a truncated cone.
According to another embodiment of the invention, the tubular walls of the first and second rotor portions are arranged axially in succession, adjacent wall ends thereof being dimensioned such that the wall end of the first rotor portion has a less diameter than the wall end of the second rotor portion. The tubular wall of the first and the second rotor portion, respectively, suitably has the shape of a truncated cone.
The inlet member is advantageously arranged to supply the fibre suspension into the first rotor portion via the base of the conical wall thereof.